A Cu2+ ion-selective fluoroionophore with dual off/on switches.

Abstract

A new malonamide fluoroionophore possessing two pyrene moieties was synthesized. This bispyrene exhibited the fluorescence of the pyrene monomer (lambda(em) = 395 nm) and intramolecular excimer (lambda(em) = 467 nm) emissions. The designed derivative showed the excellent ion sensing ability to Cu2+. The "on-off-off" and "off-on-off" fluorescence responses were demonstrated by the addition of the variable Cu2+ concentration. The utilization of the dual off/on responses could apply to the estimation of the rough Cu2+ concentration.

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PRELIMINARY COMMUNICATION
A Cu2+ion-selective fluoroionophore with dual
off/on switches
Open Access
Takayo Moriuchi-Kawakami*, Youji Hisada, Yasuhiko Shibutani
* Correspondence:
kawakami@chem.oit.ac.jp
Department of Applied Chemistry,
Faculty of Engineering, Osaka
Institute of Technology, 5-16-1
Omiya, Asahi, Osaka 535-8585,
Japan
Abstract
A new malonamide fluoroionophore possessing two pyrene moieties was synthe-
sized. This bispyrene exhibited the fluorescence of the pyrene monomer (lem= 395
nm) and intramolecular excimer (lem= 467 nm) emissions. The designed derivative
showed the excellent ion sensing ability to Cu2+. The “on-off-off” and “off-on-off”
fluorescence responses were demonstrated by the addition of the variable Cu2+con-
centration. The utilization of the dual off/on responses could apply to the estimation
of the rough Cu2+concentration.
Findings
In recent years, researches on development of ion sensing with fluorescence emission
have attracted considerable attention [1,2]. The rapid development of cation sensing is
derived from the realization of the diverse roles played by cations in biological and
chemical systems. For example, Cu2+is a significant environmental pollutant and also
an essential trace element in biological system [3]. Moreover, Cu2+is a well-known
paramagnetic ion with an unfilled d orbital and could strongly quench the fluorescence
of the fluorophore near it via electron or energy transfer [4]. In most of the reported
recognition of Cu2+, the binding of Cu2+causes quenching of the fluorescence emis-
sion [5-9]. Few reports are available for “off-on"-type fluorescent compounds that can
selectively recognize Cu2+[10-14]. However, the “off-on-off"-type sensing for Cu2+has
not been reported yet. The “off-on-off"-type sensing should be suitable for the simpli-
fied measurement, because the ion concentration could be roughly estimated.
Most of the fluoroionophore for cation recognition are composed of a cation recog-
nition site with a fluorescent moiety [15]. Pyrene is one of the most useful fluorescent
units due to their relatively efficient excimer formation and emission properties
[16-21]. Fluorescent molecules with more than one pyrene moiety exhibit not only pyr-
ene monomer emission but also intramolecular excimer emission because of strong π-
π interaction between two pyrene moieties [22,23]. If both the emission signals are uti-
lized for sensing, it would lead to the development of a novel sensing method. We
herein report dual off/on fluorescence for Cu2+ion sensing, in which both monomer
and excimer emission signals of pyrene units change effectively and cooperatively.
Therefore, the use of this dual off/on fluorescence responses could apply to estimate
the rough Cu2+concentration.
Moriuchi-Kawakami et al. Chemistry Central Journal 2010, 4:7
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© 2010 Moriuchi-Kawakami et al

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Bispyrene 1 was prepared by the synthetic routes depicted in Figure 1. Disubstituted
malonic acid dichloride was synthesized by the reaction of corresponding disubstituted
malonic acids with (COCl)2in benzene [24]. Subsequently, the reaction of disubsti-
tuted malonic acid dichlorides with 1-pyrenylmethylamine hydrochloride in benzene
gave a desired bispyrene 1 in 2% isolated yield. Derivative 2 possessing only one pyrene
moiety was also obtained with derivative 1. Bispyrene 3 was prepared by similar
method.
Derivatives 1 and 3 have two pyrene moieties as a fluorescent unit. As shown in Fig-
ure 2, bispyrenes 1 and 3 displayed both excimer (lem= 467 nm) and monomer (lem
= 395 nm) emissions when excited at 344 nm in CH3CN/CHCl3(9:1, v/v). The exci-
mer emissions were dominant compared with the monomer emissions. Derivative 2
has only one pyrene moiety as a fluorescent unit. Monopyrene 2 exhibited only typical
monomer (lem= 395 nm) emission by exciting at 344 nm in CH3CN/CHCl3(9:1, v/v).
In1H NMR spectra, the amide NH resonances of bispyrenes 1 and 3 were detected in
CDCl3at the lower field (δ 7.24 for 1 and δ 7.19 for 3) than that of the monopyrene 2
possessing only one amide group (δ 5.53). Therefore, bispyrenes 1 and 3 seem to form
intramolecular hydrogen bonds between both amide groups. The hydrogen bonds might
structurally influence intramolecular π-π interaction of two pyrene moieties, which
result in a relatively strong fluorescent in the excimer emission. Further, the excimer
emission intensity of bispyrene 1 was reduces to about half by the addition of CH3OH
(CH3CN:CHCl3:CH3OH = 99:10:1, v/v).1H NMR study in CDCl3demonstrated that the
amide NH resonance of bispyrene 1 was slightly perturbed by the addition of 10 equiv
Figure 1 Syntheses of pyrene compounds.
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and 100 equiv of CD3OD (from δ 7.24 to δ 7.29 and 7.42). These results also support the
formation of the intramolecular hydrogen bonds in bispyrene 1.
To study cation recognition properties of new malonamide-substituted pyrene deri-
vatives 1-3, the fluorescence measurements were carried out. The fluorescence emis-
sions for 1 × 10-5M solution of 1-3 in CH3CN/CHCl3(9:1, v/v) were measured in the
presence of metal perchlorates by exciting them at 344 nm. The effect of K+, Rb+, Cs+,
Mg2+, Cr3+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+and Ag+on the excimer and monomer emis-
sion intensities of 1-3 was investigated. For derivative 1, the effect of Li+, Na+, NH4+,
Ca2+, Fe2+, Cd2+and Pb2+was also examined. The addition of 1 equiv (1 × 10-5M) of
Ni2+, Cu2+, Zn2+and Ag+marginally affected the fluorescence emissions of 1. Interest-
ingly, the both monomer and excimer emission intensities of 1 were dramatically chan-
ged by the addition of 10 equiv (1 × 10-4M) of Cu2+. Figure 3 shows the intensity
ratios (I/I0) of the excimer and monomer emissions for 1 × 10-5M solution of 1 in the
presence of 1 × 10-5and 1 × 10-4M of Ni2+, Cu2+, Zn2+and Ag+ions to the excimer
and monomer emissions for 1 × 10-5M solution of 1. On the other hand, the addition
of 1 equiv (1 × 10-5M) of Cu2+caused changes in the excimer and monomer emission
intensities of bispyrene 3 significantly and selectively as illustrated in Figure 4. Bispyr-
enes 1 and 3 exhibited noteworthy responses to Cu2+as compared to other metal ions
examined. Unlike bispyrene compounds, monopyrene 2 showed nonselective response
to most of metal perchlorates, increasing or decreasing the monomer emissions.
The fluorescence responses of derivatives 1-3 to Cu2+were examined in detail. Fig-
ure 5 shows the fluorescence response of 1 × 10-5M bispyrene 1 with the addition of
Cu2+in CH3CN/CHCl3(9:1, v/v). The fluorescence spectra of 1 are characterized by
the dual emission resulting from the excimer and the monomer. Until the addition of
ca. 3 equiv (3 × 10-5M) of Cu2+, both fluorescence emissions of 1 were unaffected.
However, by the addition of 10 equiv (1 × 10-4M) of Cu2+, the fluorescence intensity
of the excimer emission decreased while the intensity of the monomer emission
Figure 2 Fluorescence spectra of derivatives 1-3 (c = 1 × 10-5M) in CH3CN/CHCl3(9:1, v/v).
Excitation wavelength was 344 nm.
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increased. Both fluorescence emissions of 1 were quenched by the further addition of
Cu2+. Thus, the addition of Cu2+to 1 gave the “on-off-off” and “off-on-off” fluores-
cence responses to the excimer and monomer emissions, respectively. The discrimina-
tions between “on” and “off” states were determined from the abrupt change of
fluorescence intensities in the excimer and monomer emissions, respectively. Bispyrene
1 exhibited two switching points that would be brought by the addition of ca. 3 and 10
equiv of Cu2+respectively.
Figure 4 Fluorescence emission changes of derivative 3 (c = 1 × 10-5M) upon addition of metal
ions K+, Cr3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+(1 equiv) in CH3CN/CHCl3(9:1, v/v). Excitation wavelength
was 344 nm.
Figure 3 Intensity ratios of the excimer (lem= 467 nm) and monomer (lem= 395 nm) emissions (I)
for 1 × 10-5M of derivative 1 in the presence of 1 × 10-5M and 1 × 10-4M of metal ions to those
emissions (I0) without the addition of metal ions.
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The fluorescence response of 1 × 10-6M bispyrene 1 with the addition of Cu2+pro-
vided the similar switching responses. Therefore, the utilization of both off/on fluores-
cence responses could apply to estimate the rough Cu2+concentration. Figure 6
illustrates the switch function of bispyrene 1. For example, the “on” signal of only the
excimer emission displays the presence of ca. 1 equiv of Cu2+and the “on” signal of only
the monomer emission indicates the presence of ca. 10 equiv of Cu2+. The “off” signals
of both emissions mean the presence of ca. 100 equiv of Cu2+. Bispyrene 3 also showed
the similar sensing ability for Cu2+. However, the off/on signals of 3 changed easily by a
little change of the concentration of Cu2+. Therefore, “on-off” responses of 1 × 10-5M of
3 were observed under the presence of over 1 equiv (1 × 10-5M) of Cu2+.
On the other hand, 1 × 10-5M of monopyrene 2 exhibited only inherent monomer
emission. The fluorescence intensity of the monomer emission of 2 linearly decreased
as the concentration of Cu2+increased to ca. 1 × 10-4M (10 equiv).
The “on-off-off” fluorescence response in the excimer emission and the “off-on-off”
fluorescence response in the monomer emission of bispyrene 1 to Cu2+could be inter-
preted as follows. The binding of Cu2+to an amide moiety [25] is considered to cleave
the intramolecular hydrogen bond resulting a conformational change to abolish intra-
molecular π-π interaction. Then the fluorescence intensity of the excimer emission
decreases while the intensity of the inherent monomer emission increases. Upon the
addition of excesses amount of Cu2+(over 10 equiv), even the inherent monomer
Figure 6 Switch function of derivative 1.
Figure 5 Fluorescence intensity changes during the titration of derivative 1 (c = 1 × 10-5M) in
CH3CN/CHCl3(9:1, v/v) with Cu2+(from 1 × 10-8to 5 × 10-4M). Excitation wavelength was 344 nm
and emission was at 467 nm for excimer and 395 nm for monomer.
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